Frequency modulation monitoring



3 Sheets-Shet 1 Filed Jan; a, .1941

SSQ Q `Jaa.; n. 6, 1942. R. J. PIERACCI FREQUENGY MODULTION MONITORINGFiled Jan. 8, 1941 5 she'elts-svheet 2 INVENTOR ATTORNEYS Jan. 6, 1942.R. J. PIERAccl FREQUENCY MODULATION MONITORING Filed Jan. s, 1941 `slsheets-sneu s Y( Rage/J Pemcc/ BY ATTO/wf: Ys

Patented Jan. 6, 1942 UNITED STATES PATENT OFFICE 2,269,126 l FREQUENCYMoDULATloN MONITORING Roger J. Pieracci, Cedar Rapids, Iowa ApplicationJanuary 8, 1941, Serial No. 373,552

8 Claims.

This invention relates to a method of monitoring frequency modulationtransmissions in radio signaling. It has for its object the provision ofmeans whereby the instantaneous frequency deviation of the transmittedwave may be visually observed so that a continuous watch on theperformance of the transmitter may be malntained. Other objects of theinvention will appear hereinafter.

Referring now to the gures which form a part of this specification, Fig.I and Fig. II show graphically the components of afrequency modulatedwave in terms of the amplitude and the distribution of the side bands inthe spectrum for various conditions of modulation. Fig. III showsschematically the arrangement of the elments.

of the invention for directly observing in an oscilloscope thecomponents of the frequency modulated wave as illustrated in Figures Iand II.

The theory of operation of the monitoring system will be understood fromthe following explanation.

It is well known that it can be shown mathematically that a frequencymodulated wave consists of a carrier and an infinite number of sidefrequencies. The expression for a frequency modulated wave is e=E1 sin(wt-i-mp sin but).

where w=21r times the carrier frequency Fc. l

p=21r times the modulating frequency Fm.

`mp=ratio of the deviation in frequency to the modulating frequency.

E1=amplitude of the unmodulated carrier wave.

the component frequencies for a few specific' cases. It will be observedthat in each case the frequency modulated wave consists of the carrierand aseries of side frequencies each spaced from the carrier and fromeach other by an amount equal to the modulating frequency. The

. by the reactance tube modulator 6.

For the modulating frequencies withl diagrams of Figures I and II areself explanatory. The arrangement of Figure III shows the means by whichthe components illustrated in Figures I and l1 may be viewed directly onan oscilloscope screen.

' Referring now to Figure III, I represents the input to the monitoringsystem from the transmission which is to be observed. 2 represents aconverter or first detector of a superheterodyne receiving system. 3represents an I. F. amplifier of 2 mgc. and 4 an amplitude modulationdetector. The output of the detector is connected across the verticalplates 8-9 of an oscilloscope Il having a screen I2, as shown. Theconverter 2, in addition to the signaling currentwhich is to be observedand which for this example is taken as a 40 mgc.i75 k.c. deviationfrequency modulated current is likewise supplied with current from a 38megacycle oscillator shown at 5. This oscillator is frequency modulatedthru k.c.

V The reactance tube modulator is controlled by a 30 cycle linear sweepvoltage from a source 1, as shown, which voltage is also applied to thehorizontal deflection plates Ill- II of the oscilloscope 0 by the leadsshown.

'Ihe operation of the system will be understood from the followingdescription with reference to.

Figure III.

'I'he FM signal (E1) to be monitored is applied from source I to theinput circuit of the converter 2. The pass band of this circuit issufficiently broad to accommodate without attenuation, a band equal toor wider than that occupied by the carrier and all of its side bands. Alocally generated FM signal (Ez) is injected into conand below thecenter frequency +100 k.c. or more by the application of a saw 'toothwave form voltage to the grid of the reactance tube modulator 6,generated by the linear sweep voltage source 1. This excursion aboveandy below the center frequency takes place at a rate approximatelyA 30cycles per second.

Assume the incoming FM signal (E1) is unmodulated and consists of only acarrier or center frequency. As the local oscillator voltage (Ez) passesthrough its center frequency, the

mixture of E1 and E2 produces a voltage at the intermediateA frequency,since it will be recalled that the difference of the center frequenciesof E1 and E2 is equal to the intermediate frequency. The output of theintermediate frequency amplier tube will be a pulse of Voltage whose`maximum value will occur at the instant the frequency difference betweenE1 and E2 is exactly equal to the intermediate frequency. The voltagewill drop s-harply about this point, when the frequency differencebetween E1 and E2 is slightly 'greater or less than the intermediatefrequency. The sharpness of the pulse is dependent on the selectivity ofthe intermediate frequency transformer (which may bea few k. c. wide)and the rate at which the local oscillator frequency is swept about itscenter frequency. It can be seen from the foregoing explanation, that ifa curve of IF output voltage vs. oscillator frequency (E2) were plotted,that a pulse similar to a s-teep resonance curve would result in theregion of i5 k. c. of the center frequency, while the region from i5 k.c. to i100 k. c. would show no output.

The sharp pulse of radio frequency voltage c from the IF amplifier 3 isapplied to the second detector 4. This is a half wave rectifier whicheliminates the negative portion of the cycle, and delivers aunidirectional pulse whose shape is the envelope of the individual RFcycles received from the IF amplifier. This sharp inverted V pulse isapplied to the vertical plates 8 9 of the oscilloscope 0 and is directlyproportional to the amplitude of the incoming FM carrier Voltage (E1) Itwould appear as a very sharp resonance curve on the screen l2 of theoscilloscope.

Assume that the FM signal to be monitored (E1) is now modulated at 175k. c. more or less, with a sinusoidal audio frequency. From FM sidebandtheory, individual discreet side bands will be produced above and belowthe carrier at frequency intervals equal to the audio frequency, andwhose amplitudes Vary in accordance with the equation of voltage for theside bands. As the local FM oscillator voltage (E2) makes its frequencyexcursion above and below its center frequency, voltages at theintermediate frequency are produced each time the frequency differenceof the side bands and local oscillator is equal to the intermediatefrequency. Proper phasing of the two'voltages is obtained by a slightadjustment of the saw tooth sweep frequency. The production of a sharpunidirectional pulse at the output terminals of the detector 4 for eachside band component occurs in the same mannery as described above forthe carrier of the unmodulated FM wave. The amplitude of each of thesepulses is directly proportional to the amplitude of the correspondingside band component.

summarizing the above description, the pulses are produced by a mixtureof two FM waves whose center frequencies differ by the intermediatefrequency. When the two FM waves are Ain properphase, .the carrier andside band comvFM signal being monitored. The several 'components areproperly spaced on the screen I2 of the oscilloscope, by applying to thehorizontal deflection plates I0-|I, the same 30 cycle linear sweepvoltage applied to the local FM oscillator.

When the electron beam is at the center of the tube ll, the linear sweepvoltage is zero as is the control voltage at the grid of the reactancetube modulator, hence the local FM oscillator 5 is at its centerfrequency, and the peak value of the carrier occurs at this point on thescreen. As the electron beam moves to the right or left of center, thefrequency of the local FM oscillator follows, and the vertical beamdeflections produced by the pulses representing the spectrum, appear onthe screen l2 in exact relation of frequency spacing with respect toeach other.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be confined to the embodimentsshown; but what I desire to cover by Letters Patent is set forth in theappended claims.

I claim:

1. The method of visually indicating the several frequency components ofa frequency modulated wave which comprises the steps of generating asecond wave having a mid-frequency differing from that of the frequencymodulated wave by a given intermediate frequency, automatically varyingthe frequency of the second wave above and below its mid-frequency in asubstantially linear manner, beating the second wave of varyingfrequency with the frequency modulated Wave to produce a series of beatfrequencies spaced apart in accordance with the frequency components ofthe frequency modulated wave and energizing a visual indicating deviceby energy derived from the said series of beat frequencies.

2. The method of visually indicating the several frequency components ofa frequency modulated wave which comprises the steps of generating asecond wave having a mid-frequency differing from that of the frequencymodulated wave by a given intermediate frequency, automatically varyingthe frequency of the second wave above and below its mid-frequency in asubstantially linear manner by an amount at least equal to the frequencydeviation of the frequency modulated wave, beating the second wave ofvarying frequency with the frequency modulated wave to produce a Seriesof beat frequencies spaced apart in accordance with the frequency-components of the frequency modulated wave, and controlling the ray ofan oscilloscope by energy derived from the said series of beatfrequencies.

3. The method of visually indicating the several frequency components ofa given frequency modulated current which comprises the steps ofgenerating a 'second current having a mid-frequency differing from thatof the given current by a desired intermediate frequency, automaticallyvarying the frequency of the second current above and below itsmid-frequency, beating the second current of varying frequency with thefrequency modulated current to produce a third current having frequencycomponents spaced apart in accordance with the frequency components ofthe frequency modulated wave, detecting the individual frequencycomponents of the third current to derive the modulation componentthereof and energizing a visual indicating device by means of thedetected modulation component.

4. Means for visually indicating the several frequency components of agiven frequency modulated current comprising, in combination, means forgenerating a second radio frequency current,

Vdicatng device comprises control means for automatically varying thefrecurrentl with the given frequency modulated current to derive a thirdcurrent of predetermined intermediate frequency, a detecting devicecon-v nected to said last named means and having an outputcircuit'arranged to pass a current corresponding to the modulationenvelope of the intermediate frequency current and a visual indicatingdevice connected to the output circuit of said detecting device. l

5. A lvisual indicating means as et forth in claim 4 in which thecontrol means for varying the frequency of the second generator arrangedto generate Ya wave `of sawtoothed shape.

6. A visual indicating means as set forth in claim ,4 in which thevisual indicating device comprises an oscilloscope having a pair ofopposed defiectng plates connected to the output circuit of thedetecting device.

7. A visual indicating means as set forth in claim 4 in which thecontrol means for varying the frequency of the second current comprisesa generator arranged to generate a current of low frequency whichincreases to a maximum value substantially linearly and in which thevisual inan oscilloscope having a first pair of deecting platesconnected tothe output circuit of the detectingdevice land a' secondpair of deflecting platesdisposed substantially normal to those of therst pair, and 'connections between vthe plates of the second pair'4 andsaid low frequency current generator.

' 8. Means for visually indicating the several frequency components of agiven frequency modulated current comprising, in combination, anoscillator for generating a second radio frequency current, a reactancetube modulator having terminals connected to said oscillator, a lowfrequency source of linear sweep voltage connected l to thegrid of saidreactance tube and. operative current comprises a to cause the reactancetube to vary the frequency i of the said second current generated bysaid oscillator over a range of frequencies at least twice as wide asthe frequency deviation of the given frequency modulated current, meansfor combining the secondl current with the given frequency modulatedcurrent to derive a third current of predetermined intermediatefrequency, a detecting devicel means and operative .to detect theindividual frequency components of the thirdy current as they areapplied successively to the detecting device, an oscilloscope having afirst pair of deflecting plates connected to the output terminals of thedetecting device and a second pair of deflecting plates disposedsubstantially normal to those of the rst pair and connections betweenthe plates of the second pair and the terminals ofA said source of sweepvoltage.

ROGER J. PIERACCI.

connected to said last named

